1. Field of the Invention
The invention relates to a switched-mode power supply (SMPS). More particularly, the invention relates to a SMPS capable of suppressing electromagnetic interference (EMI).
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a conventional SMPS using X and Y capacitors to suppress EMI. The SMPS includes a bridge rectifier, a power converter, and energy storage capacitors C1 and C2, and the power converter includes power switch components (not shown). In the SMPS, energy to be converted in each unit of time is divided into several portions by fast switching of the power switch components, and each portion of energy is converted and then transferred to the output side. When the output becomes too high or too low, the SMPS only needs to adjust the amount of the effective power in each portion.
As the switching frequency of the power switch components becomes higher, the quantity of the divided portions in each unit of time becomes larger, and the power borne in each divided portion becomes less accordingly, so that the energy borne by components in the power converter becomes less. Therefore, the components in the power converter may use a lower-level specification. In addition, a magnetic component in the power converter, such as a transformer or an inductor, induces higher voltage as the magnetic field changes faster, so that the magnetic component which operates in a higher frequency may use shorter wires to induce the required voltage and therefore may reduce the size.
Currently, high-frequency switching is a necessary manner for SMPSs. However, some loss will occur in each operation of dividing energy and will be expressed in the form of heat and noise. Low-frequency noise is conducted by wires to a live wire L and a neutral wire N of a power source on the input side of the SMPS and therefore is called conducted EMI. High-frequency noise is transmitted by radiation to space and therefore is called radiated EMI. To suppress the conducted EMI and the radiated EMI, a conventional manner is using X capacitors Cx1 and Cx2 and common-mode choke coils L1 and L2 to filter out the conducted EMI, and using Y capacitors Cy1 to Cy7 to conduct the high-frequency noise to ground to reduce the radiated EMI. The above-mentioned ground generally refers to a ground wire G of the power source on the input side of the SMPS, or a large-area metal part of an electronic product using the SMPS, such as a metal cover or a metal back bezel.
The arrangement of Y capacitors is determined according to the structure of the SMPS, because different structures generate different frequency-band noises. Moreover, the capacitances of the Y capacitors are determined according to the intensities of the noises to conduct the respective frequency-band noises to ground to reduce the radiated EMI. The Y capacitors conduct the respective noises to ground and therefore generate respective leakage currents flowing to ground. To avoid a risk of an electric shock when a user uses an electronic product, a country defines a specification about a total amount of the leakage currents of the electronic product and therefore limits a total amount of the capacitances of the Y capacitors of the SMPS used for the electronic product. However, different countries usually define different specifications about the total amount of the leakage currents, resulting in different limitations of the total amount of the capacitances of the Y capacitors. If the electronic product is sold to different countries, it needs to use the strictest specification about the total amount of the leakage currents to design the Y capacitors and results in increased design difficulty.
FIG. 2 shows schematic diagrams of (a) a snubber circuit and (b) another snubber circuit used to suppress EMI in a conventional SMPS. For the suppression of high-frequency noise, it may be implemented not only by using Y capacitors to conduct the high-frequency noise to ground, but also by electrically connecting a snubber circuit including passive components with a component generating the high-frequency noise to transform the high-frequency energy generated by the component to heat dissipated to space to reduce the radiated EMI. For example, as shown in FIG. 2 (a), a snubber circuit including a resistor R1 and a capacitor C3 is electrically connected in parallel with a power switch component, a diode D1, of the power converter of the SMPS.
As shown in FIG. 2 (b), another snubber circuit including a capacitor C4 is electrically connected in parallel with a power switch component, a power transistor Q1, of the power converter of the SMPS. However, the capacitor C4 needs to use a high-voltage resistant capacitor due to the power transistor Q1, therefore, increasing cost. Moreover, the snubber circuit needs to use a high-capacitance capacitor to limit the rate of change of voltage and current of the power switch component to achieve a better suppressive effect. However, the slow rate of change of voltage and current results that the power switch component has a rapid rise in temperature, so that the power switch component needs to use a high-temperature resistant power switch component or so that an increased size heat sink needs to be connected with the power switch component, therefore, increasing cost.